With their second album in 1991, they became serious contenders for Public Enemy's (until then) undisputed crown as hip-hop's cultural leaders. However, where their debut was propelled by a disparate array of samples, The Low-End Theory was built on a stricter musical doctrine - its spine provided by bebop jazz. Though jazz-rap crossover would become a staple of popular music in the mid-90s (Gang Starr, Jazzmatazz, etc.), this album played a substantial part in breaking down barriers between genres. Phife Dog and Q-Tip are on great form too, making the most of Quest's energetic production with cool asides and insightful observations ('The Infamous Date Rape', 'Showbusiness').